In a wavelength division multiplexed (WDM) fiber optic system, an optical signal with combined different wavelengths is simultaneously launched into a single optical fiber through an optical multiplexer. At the receiving end, each wavelength is separated and routed to individual detectors via an optical demultiplexer. WDM technology has proven to be a popular cost-effective means for increasing the carrying capacity (or bandwidth) over a single optical fiber. As the number of wavelengths (or channels) on fiber optic networks increases together with the increase of the optical network complexity, monitoring and management of such a network at the optical layer becomes increasingly more important such that the spectral characteristics of each wavelength (or channel) at key nodes in the network must be determined and analyzed in real-time. The spectral information such as channel identification, power and optical signal-to-noise ratio (OSNR) can be used for the purpose of channel inventory monitoring, real-time system error warning/alarming and signal conditioning. Not only can optical performance monitoring (OPM) serve as a vital diagnosis tool, it also provides useful feedback for controlling certain functions of optical network elements such as reconfigurable optical add/drop multiplexers (ROADM) and dynamic gain equalizers (DGE). The in-service (i.e. no interruption of data stream) optical measurement provides network operators with a fast and reliable estimate of the expected quality of service (QoS) offered to the digital clients.
While there exist a number of approaches that directly accomplish optical spectral analysis, they can be generally grouped into two configurations: (1) a tunable narrowband filter coupled with single broadband detector and (2) a fixed dispersive element followed by a detector array. The combination of these two configurations is also common.
A traditional optical spectral analyzer employs a rotating (i.e. tunable) diffraction grating so that the wavelengths are scanned across the output slit and detected by a single broadband detector. While it provides high resolution, it requires very sensitive control over rotation of the grating, resulting in a slow scanning speed and high cost.
Examples of conventional tunable narrowband filters are tunable Fabry-Perot (F-P) etalons and acousto-optic tunable filters (AOTF). The Fabry-Perot etalon is typically tuned by varying the mirror spacing whereas the AOTF is scanned by changing the RF drive frequency. The free spectral range (FSR) of a Fabry-Perot etalon presents an ambiguity problem particularly in the presence of multiple wavelengths. In addition, while both these tunable filters offer faster response time, the filter shape (and rejection ratio) remains undesirable for practical use. Last but not least, the mechanical and optical requirements imposed on F-P etalon and AOTF for achieving high stability and accuracy make these devices excessively costly.
Examples of fixed dispersive elements are commonly diffraction gratings and optical demultiplexers (e.g. arrayed waveguide gratings, and dielectric filters). In this case, input light is decomposed into its constituent wavelengths, with an appropriate resolution, and then relative intensity of each wavelength is measured using detector array (Polynkin et al., U.S. Pat. No. 6,504,976). The same concept has also been demonstrated using a blazed fiber Bragg grating (FBG). The fundamental limitation of this kind of approach is that it is very difficult to extract accurate OSNR information for tight channel spacing. One of the drawbacks of using an optical demultiplexer as the dispersive element is that the power measurement is prone to errors when there is thermal-wavelength drift of the spectral element due to the fact that optical demultiplexer gives a set of fixed discrete channels with a predefined frequency interval (channel spacing).
Alternatively, since a detector array is usually expensive, particularly for high spatial resolution, a tunable spatial filter (acting as a wavelength selector) can be used between the dispersive element and a single detector (Marianik et al., U.S. Pat. No. 5,305,083). Such apparatus may include micro shutters, micro mercury switches, an array of micro electromechanical mirrors or a liquid crystal display array. One of the interesting features of this hybrid configuration is that it turns a conventional scanning monochromator into a random access monochromator.
Another example of tunable narrowband filters 22 is tunable fiber Bragg gratings (FBGs). Since the filter is an in-fiber device, it has many distinct advantages over traditional bulk optical components. These include low insertion loss, low polarization dependent loss, low polarization mode dispersion and so on. What is more, an FBG offers near-ideal filter characteristics for the purpose of selecting a single channel and rejecting all others (Alavie et al., U.S. Pat. No. 6,310,703). The common means of tuning an FBG are based on applying mechanical strain and temperature. However, temperature tuning is less attractive due to the slow thermal response and high temperature required. To cover the entire C or L spectral band, strain tuning suffers from the fact that it requires excessively high strain (˜3%) so that long-term reliability of the fiber itself becomes a concern. It is also a nontrivial task to mechanically tune an FBG over such a wide spectral range without compromising the tuning speed.
Another FBG-based method of creating and tuning a narrow bandpass filter involves the interaction between a traveling acoustic pulse and a broadband-chirped FBG (Asseh et al., U.S. Pat. No. 6,510,256). Since different wavelengths are reflected at different physical portions of the grating, a narrow transmission window is spectrally opened wherever the longitudinal acoustic pulse is present within the stop-band of the grating due to the temporary collapse of the local Bragg coupling condition. However, there are a few technical difficulties. Firstly the chirped FBG must be very strong (i.e. long grating length and high reflectivity) in order to achieve good channel isolation. Secondly a very high speed and precision electronics circuit has to be used in order to capture the acoustic pulse. Finally coupling of the acoustic pulse into fiber requires tight and delicate control. Solving these technical issues usually means associated high cost.
Since the efficiency and cost of operation of optical networks are becoming increasingly important, what is needed is a cost-effective design for the practical use of optical performance monitoring.